Many conventional buckle devices are provided with a switch means for detecting whether or not a tongue is located in the lock status with the latch piece of the buckle device. However, these devices are switch-activated through a mechanism which utilizes the movement of a tongue, or the movement of a slider which is in turn moved by means of the tongue, and can only detect whether the tongue is located in the buckle device or not. Therefore, when the tongue is inserted into the buckle device, consideration must be always given to the so-called "false latch" or "partial engagement" status wherein the tongue and latch piece are not correctly engaged. Therefore a system is desired wherein it is possible to reliably detect that the tongue and latch piece are securely engaged or interlocked.